CN118330996A - Method for regulating and controlling micro lens surface shape by utilizing additional pressure - Google Patents

Abstract

The present invention belongs to the field of micro-nano optical device preparation, and discloses a method for controlling the surface shape of a microlens by using an additional pressure, comprising the following steps: (1) coating a photoresist layer on a substrate and forming a photoresist column structure; (2) forming a photoresist structure with a curved surface shape by a thermal reflow process through the photoresist column structure on the substrate, and applying an additional pressure through a pressure control device during the thermal reflow process, thereby adjusting the curved surface shape of the photoresist structure; (3) using an etching process to transfer the curved surface shape of the photoresist structure to the substrate, thereby obtaining a microlens. The present invention actively controls the external environmental pressure of the photoresist during thermal reflow by introducing a pressure control device, and the curved surface shape formed by the photoresist during the thermal reflow process is regulated by the additional pressure, which is equivalent to introducing a new control parameter during the thermal reflow process of the photoresist, providing a new way for surface shape control, and is easy to operate and flexible to control.

Description

A method for controlling the surface shape of microlens by using additional pressure

Technical Field

The present invention belongs to the field of micro-nano optical device preparation, and more specifically, relates to a method for controlling the surface shape of a microlens by using additional pressure, which is helpful for obtaining microlenses with different curvature radii.

Background technique

A microlens is a lens structure with micrometer-level size. Compared with traditional lenses, microlenses have advantages such as small size and easy integration, and can realize basic lens functions and more practical functions, such as optical computing, optical data transmission, optical array imaging, etc. At present, the main microlens preparation technologies include holography, planar process ion exchange, Fresnel zone lens method, photosensitive glass method and photoresist thermal reflow technology, among which photoresist thermal reflow technology has become one of the most popular methods in the field of microlens preparation due to its advantages of fast, simple and low cost.

However, the main problem in preparing microlenses using photoresist thermal reflow technology is that the contact angle between the photoresist and the substrate after hot melting determines the surface shape of the microlens, which is closely related to the properties of the photoresist and the properties of the substrate surface, and has nothing to do with the thickness of the photoresist and the radius of the microlens. This limits the surface shape and application range of the microlens formed after hot melting of the photoresist.

Chinese patent CN202210911073.0 discloses a method of changing the hydrophilicity and hydrophobicity of the substrate surface through a nitrogen-oxygen ternary compound interface layer, so that the contact angle α of the subsequent photoresist column with the substrate during thermal reflow can be continuously adjusted within a certain range, thereby making the surface shape of the prepared microlens continuously adjustable within a certain range, which helps to obtain microlenses with different curvatures. However, as an organic polymer with a relatively complex molecular structure, photoresist has both hydrophilic and hydrophobic groups on its molecular chain in most cases, which limits the types of photoresists suitable for the above-mentioned technology, and also limits the range of surface shapes that can be obtained by adjusting the hydrophilicity and hydrophobicity of the substrate surface.

Summary of the invention

In view of the above defects or improvement needs of the prior art, the purpose of the present invention is to provide a method for controlling the surface shape of a microlens by using additional pressure, wherein the external environmental pressure of the photoresist is actively regulated by introducing a pressure control device during hot reflow, thereby changing the pressure difference (corresponding to the additional pressure) inside and outside the photoresist, and the additional pressure is regulated by the pressure control device, thereby changing the curvature radius of the curved surface formed during the hot reflow of the photoresist melt. The present invention introduces additional pressure, and the curved surface shape formed by the photoresist during the hot reflow process is regulated by the additional pressure, which is equivalent to introducing a new regulating parameter during the hot reflow process of the photoresist, providing a new way for surface shape regulation, and is easy to operate and flexible to regulate, which helps to obtain microlenses with different curvature radii.

To achieve the above object, according to one aspect of the present invention, a method for controlling the surface shape of a microlens by using additional pressure is provided, characterized in that it comprises the following steps:

(1) coating a photoresist layer on a substrate, and forming a photoresist column structure by photolithography and development;

(2) forming a photoresist structure having a curved surface by a thermal reflow process on the photoresist column structure on the substrate, and applying additional pressure by a pressure control device during the thermal reflow process to adjust the curved surface of the photoresist structure; and then cooling;

(3) Using an etching process, the substrate obtained in step (2) and the photoresist structure with a curved surface on the substrate are etched to transfer the curved surface of the photoresist structure to the substrate, thereby obtaining a microlens.

As a further preferred embodiment of the present invention, in step (2), the pressure control device applies the additional pressure by vacuuming or by additional pressurization;

The substrate and the photoresist glue column structure on the substrate are placed in a pressure control device capable of adjusting the internal gas pressure, and are heated by a heating component located outside or inside the pressure control device to perform a thermal reflow process.

As a further preferred embodiment of the present invention, in step (2), the additional pressure applied by the pressure control device is used to provide a pressure of 0.1×10 ^-7 to 0.1×10 ³ MPa to the photoresist structure with a curved surface in the thermal reflow process.

As a further preferred embodiment of the present invention, in step (1), the photoresist column structure is a periodic array;

Correspondingly, the microlenses obtained in step (3) are microlens arrays.

As a further preferred embodiment of the present invention, in step (1), the substrate is selected from silicon, germanium, glass, quartz, and sapphire.

As a further preferred embodiment of the present invention, in step (1), the thickness of the photoresist column structure is 5 μm to 100 μm;

The cross-sectional size of the photoresist column structure is 50 μm to 1500 μm.

As a further preferred embodiment of the present invention, in step (3), the etching process is to etch the substrate and the photoresist in equal proportions.

As a further preferred embodiment of the present invention, in step (3), the etching process is specifically a reactive ion etching process;

Preferably, the reactive ion etching process is specifically reactive ion etching (RIE) or inductively coupled reactive ion etching (ICP-RIE);

More preferably, the reactive ion etching process is inductively coupled reactive ion etching (ICP-RIE), and the etching gases are _CF4 and Ar.

As a further preferred embodiment of the present invention, in step (1), the photoresist used for the photoresist layer is AZ10XT photoresist;

In step (2), the heating temperature used in the heat reflux process is 130-160° C., and the heating time is not less than 5 minutes.

Through the above technical scheme conceived by the present invention, compared with the prior art, the present invention introduces additional pressure by using a pressure control device in the hot reflow technology, changes the pressure difference inside and outside the photoresist under the action of the pressure control device, actively regulates the additional pressure inside and outside the photoresist during hot reflow, thereby changing the curvature radius of the photoresist surface during hot reflow, and by adjusting the size of the additional pressure, the curvature radius of the photoresist surface can be controlled, and then its surface shape can be regulated, and the surface shape of the microlens can be further controlled. In addition, in the etching stage, the etching process can be optimally controlled so that the photoresist and the substrate are etched at an equal etching rate of 1:1. In this way, when the photoresist etching is completed, the curved surface shape of the photoresist before the etching begins will be transferred to the substrate at an equal ratio of 1:1 to obtain a microlens. The present invention uses additional pressure to control the hot reflow process to form the curved surface shape of the photoresist melt, introduces pressure as a new control parameter in the process of shrinkage and deformation of the photoresist melt, provides a new way for surface shape control, and is easy to operate and flexible to control, which helps to obtain microlenses with different curvature radii.

When the photoresist thermal reflow technology is used to prepare the microlens, the contact angle between the photoresist and the substrate after the thermal reflow determines the surface shape of the microlens. The thermal reflow technology uses the effect of surface tension during the melting process of the photoresist to minimize the surface area of the photoresist to form a spherical lens shape. The surface tension includes three parts: the surface tension between the photoresist and the surrounding air, the surface tension between the photoresist and the solid substrate, and the surface tension between the substrate and the surrounding air. When the photoresist, the substrate and the surrounding atmosphere are fixed, the surface tension is a constant. This makes the microlens prepared by the thermal reflow technology show an obvious critical angle phenomenon, and the forming range of the surface curvature is greatly limited (that is, when the traditional process adopts the photolithography hot melt method to make a microlens, after the photoresist and the substrate are selected, the surface shape of the photoresist formed by the hot melt method is fixed in principle, which greatly limits the surface shape of the microlens and its further application). In order to overcome this problem, the present invention uses a pressure control device to change the pressure difference inside and outside the photoresist during the process of forming the microlens using the thermal reflow technology. Based on the principle of capillary phenomenon, the critical angle phenomenon can be broken through, thereby regulating the contact angle between the photoresist and the substrate after thermal reflow. The present invention introduces additional pressure, which is equivalent to introducing a new control parameter in the process of shrinkage and deformation of the photoresist melt. Therefore, by controlling the additional pressure, a microlens with a continuously adjustable surface shape over a wide range can be obtained (of course, it can also be further combined with the temperature control strategy known in the prior art and the substrate hydrophilicity control strategy).

The photoresist hot reflow technology reported in the prior art has strategies for controlling the surface shape by regulating the temperature, and also strategies for controlling the surface shape by adjusting the hydrophilicity of the substrate surface to control the contact angle between the photoresist and the substrate after hot melting, but they are often carried out under normal pressure (0.1MPa); the present invention is the first to use pressure as a controllable variable, using vacuum or additional pressure to control the pressure to change the morphology of the photoresist lens formed by hot reflow, providing a new means for controlling the morphology. Compared with the prior art such as Chinese patent CN202210911073.0, the method of the present invention that uses additional pressure to control the formation of curved surface shape during hot reflow is not affected by the hydrophilic or hydrophobic groups of the photoresist itself, and has stronger universality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a cross-sectional structure of a substrate provided by the present invention.

FIG. 2 is a schematic diagram of a cross-sectional structure of a substrate spin-coated with photoresist before photolithography provided by the present invention.

FIG3 is a schematic diagram of a cross-sectional structure after photolithography provided by the present invention.

FIG. 4 is a schematic cross-sectional view of the heat reflow process provided by the present invention.

FIG5 is a schematic diagram of the cross-sectional structure of a microlens obtained after reactive ion etching provided by the present invention.

FIG6 is a schematic diagram of the planar structure of the spherical surface photoresist after the thermal reflow process provided by the present invention.

FIG. 7 is a graph showing the actual measurement of the thermal reflow step profiler curve of Comparative Example 1 (using a normal pressure environment of 0.1 MPa) (the unit of the horizontal axis is μm).

FIG. 8 is a graph showing a measured thermal reflow step profiler curve of Example 1 of the present invention (using a 0.1×10 ^-5 MPa vacuum environment) (the unit of the abscissa axis is μm).

In FIGS. 1 to 6 , the white area represents the photoresist, and the gray area represents the substrate; the meanings of the reference numerals in the figures are as follows:

101 Substrate

201 Photoresist spin-coated on substrate

201-1 Cylindrical photoresist column (after development)

201-2 Photoresist with spherical structure with curved surface (formed by thermal reflow)

201-3 Microlens (after etching)

301 Pressure Control Device

Detailed ways

In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

It should be noted that the illustrations provided in this embodiment are only used to schematically illustrate the basic concept of the present invention. The illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components actually implemented. In actual implementation, the type, quantity and proportion of each component may be changed, and the layout of the components may also be more complicated.

Embodiment 1:

As shown in Figures 1, 2, 3, 4, 5 and 6, the present embodiment uses additional pressure to control the microlens surface shape and includes at least: a substrate 101, and a photoresist 201 spin-coated on the substrate; wherein the photoresist is successively subjected to ultraviolet lithography to form a cylindrical photoresist column 201-1 and subjected to thermal reflow to form a photoresist 201-2 with a spherical structure having a curved surface shape.

This embodiment includes the following steps:

Step S1, providing a substrate 101 such as silicon, germanium, etc., which is known in the prior art to be used as a microlens material;

The thickness of the substrate should be greater than the thickness of the photoresist to ensure that the subsequent photoresist microlens is patterned by ion beam etching;

Optionally, in step S1, the substrate material is silicon, germanium, glass, quartz, sapphire, etc., which can be used as a microlens material;

In this embodiment, the substrate material is silicon, and its thickness is 500 μm. Before spin coating the photoresist, the surface of the silicon wafer is cleaned with acetone, isopropanol and water, as shown in FIG1 .

Step S2, then use a coating machine to evenly spin-coat the photoresist on the provided substrate with certain parameters, and pattern the photoresist by ultraviolet exposure (which can be exposed by an ultraviolet photolithography machine), and form a cylindrical photoresist column structure after development, specifically:

Spin-coat the provided substrate with photoresist by using a spin coater, then expose it to ultraviolet light, and then soak it in a developer to remove the denatured photoresist, so that the photoresist is patterned;

In this embodiment, photoresist AZ10XT is selected, the spin coater speed is 600rpm, the time is 90s, and a 30μm photoresist coating is formed, that is, the photoresist 201 spin-coated on the substrate, as shown in FIG2 , and then subjected to ultraviolet exposure, the dose is set to 950mJ/cm ² , and then immersed in developer AZ 300MIF for 6min to form a patterned photoresist, that is, a cylindrical photoresist column 201-1, as shown in FIG3 .

Step S3, heating the substrate with the photoresist column at a certain temperature so that the photoresist column is thermally refluxed to form a photoresist microlens surface shape, and at the same time, the morphology thereof is regulated by the pressure control device 301, specifically:

The patterned cylindrical photoresist is placed on a hot plate, and the cylindrical photoresist is thermally reflowed to form a curved surface photoresist structure. The heating temperature and time vary depending on the type of photoresist. During the thermal reflow process, the additional pressure on the surface of the photoresist is changed by a pressure control device to control the morphology of the photoresist.

In this embodiment, according to the selected photoresist AZ10XT, the heating temperature is 130° C. and the heating time is 10 min, so that the photoresist is thermally refluxed to form a photoresist 201 - 2 with a spherical structure having a curved surface, as shown in FIG. 4 .

In this embodiment, the photoresist thermal reflow process is carried out in a pressure control device capable of adjusting the internal gas pressure. By adjusting the internal gas pressure of the device, the internal gas pressure is obtained to be 0.1×10 ^-7 to 0.1×10 ³ MPa (that is, the pressure control device can be either a vacuum device or a pressurizing device for additional pressure application). The curved surface photoresist structure is regulated by the additional pressure during the formation process, as shown in FIG4 .

After the photoresist 201 - 2 with a spherical structure having a curved surface meets the preset shape requirement, it can be cooled first, and then the pressure control device can be adjusted to restore the normal pressure environment in the cavity.

Step S4, using reactive ion etching technology to etch the photoresist with the curved surface structure, so that the pattern is transferred to the substrate to form a microlens, specifically:

The patterned photoresist that has been heat-reflowed is used as a mask, and an etching process is used to achieve a 1:1 etching rate between the photoresist and the substrate at the bottom (that is, the etching selectivity ratio between the photoresist and the substrate is 1:1; of course, considering that the etching ratio is also a means of adjusting the surface shape in the later stage, the present invention can also be combined with the etching ratio control means for regulation. At this time, the etching selectivity ratio between the photoresist and the substrate can be flexibly adjusted as needed). When the photoresist is completely etched, the pattern of the photoresist microlens is successfully transferred to the substrate material;

Optionally, in step S4, the reactive ion etching technology used for transferring the microlens pattern may be reactive ion beam etching (RIE) and inductively coupled reactive ion beam etching (ICP-RIE), and ICP-RIE with a higher selectivity and smoothness is preferably used for etching;

This example uses an inductively coupled reactive ion beam etching process, with reactive etching gas flow rates and types of 50 sccm _CF4 and 10 sccm Ar, ICP power of 2000 W, HF power of 250 W, and pressure in the etching chamber of 30 mtorr. For a 70 μm thick photoresist, the etching time is 60 min, so that the photoresist can be fully etched and the pattern is successfully transferred to the substrate to form a microlens structure, as shown in FIG5 .

In this embodiment, a vacuum environment of 0.1×10 ^-5 MPa is used in step S3 , and the corresponding result is shown in FIG. 8 .

Comparative Example 1

The only difference between this comparative example and Example 1 is that in step S3, the pressure control device 301 is not used, but the normal pressure (0.1 MPa) environment is directly used. The corresponding results are shown in FIG7 .

Comparing Figures 7 and 8, it is not difficult to see that the curvature of the formed microlens structure can be controlled by the pressure of the thermal reflow process. Increasing the air pressure of the pressure device will reduce the radius of curvature of the photoresist lens morphology formed during thermal reflow; reducing the air pressure of the pressure device will increase the radius of curvature of the photoresist lens morphology formed during thermal reflow. It can be adjusted flexibly according to actual needs by vacuuming (at this time, the pressure control device 301 will provide a pressure condition of <0.1MPa) or additional pressurization (at this time, the pressure control device 301 will provide a pressure condition of >0.1MPa).

The above embodiments are only examples. For example, the thickness of the photoresist spun on the substrate can be flexibly adjusted according to actual needs (for example, the thickness can be controlled by changing the rotation speed, time and number of spin coating of the spin coater); the diameter of the photoresist column after photolithography and development can also be flexibly adjusted according to actual needs (for example, the diameter of the photoresist column after photolithography and development can be between 50 μm and 1500 μm); the shape of the photoresist column, in addition to the circular column, can also be an elliptical column, a square column, etc., which can be flexibly adjusted according to the projection shape of the subsequent lens, and their shapes do not affect the use of pressure to regulate the curved surface shape in the present invention; for another example, the photoresist thermal reflow process uses different heating temperatures and heating times according to the type and thickness of the photoresist, as long as the photoresist thermal reflow process is fully carried out, so that the photoresist is fully melted and shrinks under the action of surface tension to form a curved surface; in addition, the heating method used for thermal reflow, in addition to hot plate heating, can also use other heating methods (such as RF radio frequency heating, laser heating, etc.). Of course, in addition to the single photoresist column structure illustrated in the example, multiple photoresist column structure arrays can also be arranged on the same substrate to obtain a microlens array accordingly.

It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (9)

1. A method for controlling the surface shape of a microlens by using additional pressure, characterized in that it comprises the following steps: (1) coating a photoresist layer on a substrate, and forming a photoresist column structure by photolithography and development; (2) forming a photoresist structure having a curved surface by a thermal reflow process on the photoresist column structure on the substrate, and applying additional pressure by a pressure control device during the thermal reflow process to adjust the curved surface of the photoresist structure; and then cooling; (3) Using an etching process, the substrate obtained in step (2) and the photoresist structure with a curved surface on the substrate are etched to transfer the curved surface of the photoresist structure to the substrate, thereby obtaining a microlens. 2. The method according to claim 1, characterized in that in step (2), the pressure control device applies the additional pressure by vacuuming or by additional pressurization; The substrate and the photoresist glue column structure on the substrate are placed in a pressure control device capable of adjusting the internal gas pressure, and are heated by a heating component located outside or inside the pressure control device to perform a thermal reflow process. 3. The method according to claim 1, characterized in that in step (2), the additional pressure applied by the pressure control device is used to provide a pressure of 0.1×10 ^-7 to 0.1×10 ³ MPa to the photoresist structure with a curved surface in a thermal reflow process. 4. The method according to claim 1, characterized in that in step (1), the photoresist column structure is a periodic array; Correspondingly, the microlenses obtained in step (3) are microlens arrays. 5. The method according to claim 1, characterized in that in step (1), the substrate is selected from silicon, germanium, glass, quartz, and sapphire. 6. The method according to claim 1, characterized in that in step (1), the thickness of the photoresist column structure is 5 μm to 100 μm; The cross-sectional size of the photoresist column structure is 50 μm to 1500 μm. 7. The method as claimed in claim 1, characterized in that in step (3), the etching process is to etch the substrate and the photoresist in equal proportions. 8. The method according to claim 1, characterized in that in step (3), the etching process is specifically a reactive ion etching process; Preferably, the reactive ion etching process is specifically reactive ion etching (RIE) or inductively coupled reactive ion etching (ICP-RIE); More preferably, the reactive ion etching process is inductively coupled reactive ion etching (ICP-RIE), and the etching gases are _CF4 and Ar. 9. The method according to claim 1, characterized in that, in step (1), the photoresist used for the photoresist layer is AZ10XT photoresist; In step (2), the heating temperature used in the heat reflux process is 130-160° C., and the heating time is not less than 5 minutes.